A TDI system operates to synchronize the operation of an imaging system with a motion of a feature of interest in a scene viewed by the imaging system in order to increase the integration time and, hence, increase the sensitivity of the imaging system.
It is known that a degradation of a TDI system Modulation Transfer Function (MTF) results from a mismatch between a velocity of an optical image and that of a corresponding charge image formed within a TDI imaging array. This effect is commonly referred to as velocity-mismatch MTF degradation.
In conventional TDI systems, the velocity with which the charge image is moved in the "along-track" direction (the TDI rate) is determined by apriori estimates of the velocity of the optical image, or by indirect measurements of the velocity. This is essentially an open-loop system solution, and any errors that may occur in the estimate of optical image velocity result in the velocity-mismatch MTF degradation referred to above.
As one example, the TDI system may be mounted within a rotating satellite platform and may view a scene through an aperture within a wall of the satellite. An estimate of, and/or a measurement of, the satellite rotational rate is employed to move a desired charge image, such as an image of a star or some other celestial object, across a TDI imaging array. Movement of the charge image is accomplished by clocking the TDI imaging array so as to cause the charge image to move from row to row in the TDI "along-track" direction. However, if the estimate and/or measurement of the satellite rotational rate is in error, then the movement of the charge image of the star does not accurately follow the movement of the actual star image that results from satellite rotation. This difference between the TDI velocity and the actual velocity of image motion results in the degradation of the MTF of the TDI imaging system.
It is thus one object of this invention to provide a TDI imaging system with an improved MTF by substantially eliminating velocity-mismatch MTF degradation.
It is another object of this invention to provide a closed-loop TDI imaging system wherein the velocity of the charge image (TDI velocity) is adaptively controlled in order to maximize a velocity match metric, and wherein the system requires no apriori knowledge of the velocity of the TDI image.